1. Field of the Invention
The present invention relates to an overhead camshaft type valve train for an internal combustion chamber which includes rocker arms for driving engine valves, and more particularly to an overhead camshaft type valve train including biasing means for biasing the rocker arms to either a cam side or an engine valve side.
2. Description of the Related Art
Conventionally, as an overhead camshaft type valve train including rocker arms for driving engine valves, Japanese Utility Model Publication No. 34563/1994 discloses an overhead camshaft type valve train for an internal combustion engine having biasing members for biasing the rocker arms to a cam side. This overhead camshaft type valve train includes intake side and exhaust side valve trains having the same construction. The intake side valve train for each cylinder includes a pair of drive rocker arms, a free rocker arm and a connection switching mechanism. The pair of drive rocker arms are brought into sliding contact with two low-speed cams, respectively, provided on a camshaft rotatably supported by cam holders fastened to a cylinder head and are interlockingly connected to a pair of intake valves, respectively. The free rocker arm is situated between the two drive rocker arms to be brought into sliding contact with a high-speed cam. The connection switching mechanism switches over the connection and release of connection of the drive rocker arms and the free rocker arm. Since the free rocker arm moves idly irrespective of the intake valve when the engine is driven at low speeds, in order to prevent the rough action thereof, a lost motion mechanism (corresponding to the biasing member) is provided on the intake side valve train for biasing the free rocker arm to the high speed cam side, and the lost motion mechanism is held in a holding portion having a recessed portion formed in the cylinder head.
Incidentally, according to the related art, since the holding portion for the lost motion mechanism is formed in the cylinder head, the construction of the cylinder head becomes complicated in which fixing portions for valve guides for slidingly guiding the intake valve and the exhaust valve and accommodation cylinders for accommodating spark plugs are formed, and there is caused a problem that productivity is decreased. In addition, since a space for the holding portion needs to be secured, the compact layout of the fixing portions, the accommodating cylinders and moreover valve springs becomes difficult, and this causes a further problem that the cylinder head is enlarged, this leading to the enlargement of the valve train chamber and eventually the internal combustion engine. To cope with this, in order to simply the construction of the cylinder head without forming the holding portions in the cylinder head so as to improve the productivity and realize a compact layout of the fixing portions, the accommodating cylinders and the valve springs, it is contemplated that a member is separately prepared in which a holding portion is formed for attachment to the cylinder head. However, the valve train chamber and hence the internal combustion engine are enlarged to such an extent that the additional member is attached thereto, and in the event that the internal combustion engine is enlarged, the weight thereof is increased. Moreover, the number of components is increased and hence the assembling man hours are also increased, leading to the deterioration of assembling performance.
On the other hand, a load is exerted to the holding portion for holding the lost motion mechanism, from the free rocker arm which is rocked when brought into sliding contact with the high speed cam. Therefore, it is desirable that the deformation of the cylinder head in which the holding portions are formed due to the load so exerted is made as small as possible and that a biasing force which is stable in a certain direction is exerted to the free rocker arm. Accordingly, the thickness of a peripheral portion of the member where the holding portion is formed needs to be increased, or reinforcement ribs are formed in such a manner as to connect to the holding portion in the peripheral portion of the holding portion so that the rigidity can be increased. As a result of such efforts, the cylinder head is made large in size and heavy in weight, thus causing a problem increasing the size and weight of the internal combustion engine.
Further, U.S. Pat. No. 5,297,506 discloses a valve operating system in which a holding portion for holding a spring is provided in a cam holder. However, according to U.S. Pat. No. 5,297,506, the holding portion is protruded from the cam holder, thus causing a problem increasing the size of the internal combustion engine.
The invention was made in view of the situations, and an object of the invention is to reduce the size and weight of a cylinder head on which an overhead camshaft type valve train having biasing members held at holding portions is provided, leading eventually to provision of an overhead camshaft type valve train for an internal combustion engine which can reduce the size and weight of the internal combustion engine.
With a view to attaining the object, according to a first aspect of the invention, there is provided an overhead camshaft type valve train for an internal combustion engine including: a camshaft supported by a plurality of holders provided on an cylinder head; rocker arms mockingly supported on a rocker shaft for driving engine valves to open; cams provided on the camshaft for regulating the rocking action of the rocker arms; and biasing members for biasing the rocker arms to either a cam side or an engine valve side, wherein at least one of the plurality of holders is fastened to the cylinder head with fastening members at two fastening portions spaced apart from each other in an orthogonal direction which intersects at right angles with an axial direction of the rocker shaft when viewed from a plane, and wherein the at least one of holders includes a holding portion for holding the biasing member, which is integrally formed on at least one of the two fastening portions.
According to the first aspect of the invention, the following advantage can be provided. Namely, the holding member for holding the biasing member is formed on the holder fastened to the cylinder head for supporting the shaft member which is a constituent component of the valve train, the construction of the cylinder head is simplified to improve the productivity of cylinder heads. Moreover, since there is no need to prepare a separate member for forming the holding portion for attachment to the cylinder head, the number of components is reduced to improve the assembling efficiency of internal combustion engines, and at the same time the size and weight of an internal combustion engine so produced can be reduced. Furthermore, since the fastening portion where the holding portion is formed integrally is a portion where the holder with the holding portion is fastened to the cylinder head with the fastening member, the deformation generated in the fastening portion where the holding portion is formed by virtue of a load exerted from the rocker arm to the holding portion via the biasing member is extremely small, and the biasing force which is stable in the certain direction can be exerted to the rocker arm. Thus, since the deformation amount of the holder with the holding portion due to the load exerted to the holding portion is made as mall as possible by making use of the fastening portion, there is almost no need to form a thicker portion on the holder or reinforcement ribs that would otherwise be needed due to the formation of the holding portion on the holder, whereby the holder with the holding portion can be made small in size and light in weight, this leading to an internal combustion chamber which is small in size and light in weight. Moreover, since the holding portion is integrally formed on the fastening portion, the rigidity thereof can be improved further.
According to a second aspect of the invention, there is provided an overhead camshaft type valve train for an internal combustion engine according to the first aspect of the invention, wherein the rocker arm is disposed between two of the holders which are adjacent in the axial direction, wherein at least one of the two holders is the holder including the holding portion, and wherein a center axis of the fastening member at the one of the fastening portions on the one of the two holders is situated on an opposite side to a side where the holding portion is situated, in relation to a reference straight line passing through a center point in the axial direction between a center axis of the fastening member at the one of the fastening portions on the one of the two holders and a center axis of the fastening member at the other fastening portion of the two fastening portions as viewed from the plane and being parallel to the orthogonal direction.
According to the second aspect of the invention, in addition to the advantage provided by the first aspect of the invention, the following advantage can be provided. Namely, in the holder with the holding portion which is one of the two holders provided in such a manner as to hold therebetween the rocker arm, since the center axis of the fastening member at the fastening portion where the holding portion is formed is situated on the opposite side to the side where the holding portion is formed in relation to the reference straight line, the space in the axial direction between the holding portion and the other holder can be increased accordingly, and therefore, the sufficient space for disposition of the rocker arm can be secured without increasing the space between the two holders in the axial direction, namely, without increasing the width of the cylinder head in the axial direction while maintaining the small size and light weight of the cylinder head.
According to the third aspect of the invention, there is provided An overhead camshaft type valve train for an internal combustion engine including: a camshaft supported by a plurality of holders provided on an cylinder head; drive rocker arms rockingly supported on a rocker shaft in such a manner as to be interlockingly connected to respective engine valves comprising an intake valve and an exhaust valve so as to drive the engine valves to open; free rocker arms supported mockingly on the rocker shaft in such a manner as to have a free condition relative to the engine valves; cams provided on the camshaft for regulating the rocking actions of the drive rocker arms and the free rocker arms; connection switching mechanisms for switching over the connection and the release of connection of the drive rocker arms and the free rocker arms; and biasing members for biasing the free rocker arms to a cam side, wherein at least one of the plurality of holders is fastened to the cylinder head with fastening members at two fastening portions spaced apart from each other in an orthogonal direction which intersects at right angles with an axial direction of the rocker shaft when viewed from a plane, wherein the at least one of holders includes a holding portion for holding the biasing member, which is integrally formed on at least one of the two fastening portions, and wherein the free rocker arm is disposed closer to the holder including the holding portion in the axial direction than the drive rocker arm.
According to the third aspect of the invention, in the overhead camshaft type valve train for driving the engine valves comprising the intake valves and exhaust valves, in addition to the advantage provided by the first aspect of the invention, the following advantage can be provided. Namely, since the free rocker arm is disposed in the vicinity of the holder with the holding portion in the axial direction, the protruding amount of the holding portion for holding the biasing member for biasing the free rocker arm from the holder may be small. Therefore, in this respect, too, the holder is made small in size and light in weight, and hence the internal combustion engine is made small in size and light in weight.
According to a fourth aspect of the invention, there is provided an overhead camshaft type valve train for an internal combustion chamber according to any of the first to third aspects of the invention, wherein a side wall of the holding portion extends along a fastening direction of the fastening member.
According to the fourth aspect of the invention, in addition to the advantages provided by the aspects of the invention so cited, the following advantage can be provided. Namely, since the side wall of the holding portion is formed in such a manner as to extend along the fastening portion along the fastening member, the connecting range between the holding portion and the fastening portion can be set large in the fastening direction, whereby the advantage provided by the first aspect of the invention can be improved further.
According to a fifth aspect of the invention, there is provided an overhead camshaft type valve train for an internal combustion engine according to any of the first to fourth aspects of the invention, wherein the holding portions are formed on the two fastening portions at positions which hold the shaft member therebetween and wherein one of the holding portions is situated on one side of the holder in the axial direction whereas the other holding portion is situated on an opposite side of the holder in the axial direction.
According to the fifth aspect of the invention, in addition to the advantages provided by the aspects of the invention so cited, the following advantage can be provided. Namely, since the load exerted from the rocker arm pushed by the cam to the two holding members is allowed to be exerted to the holder with the holding portion at the positions which are spaced apart in the orthogonal direction in such a manner as to hold therebetween the shaft member, as well as to the both sides thereof in the axial direction, the points to which the load is exerted on the holder can be dispersed to reduce the stress that would be otherwise generated by the load, this facilitating the design of the holder and also increasing the durability of the holder.
According to a sixth aspect of the invention, there is provided an overhead camshaft type valve train for an internal combustion engine according to any of the first to fifth aspects of the invention, wherein a recessed portion is formed between the fastening portion and the holding portion.
According to the sixth aspect of the invention, in addition to the advantages provided by the aspects of the invention so cited, the following advantage can be provided. Namely, since the recessed portion is formed between the fastening portion and the holding portion, the holder with the holding portion is made light in weight, this eventually leading to an internal combustion engine which is made light in weight.
According to a seventh aspect of the invention, there is provided an overhead camshaft type valve train according to any of the first to sixth aspects of the invention, wherein the holding portion on the holder with a holding portion protrudes in the axial direction from one of sides, as viewed from the plane, of the holder with a holding portion situated between the two fastening portions, and wherein part of a holding hole formed in the holding portion for accommodation of the biasing member is situated closer to the other side of the holder in the axial direction than the one of sides of the holder.
According to the seventh aspect of the invention, in addition to the advantages provided by the aspects of the invention so cited, the following advantage can be provided. Namely, since part of the holding hole is situated closer to the other side of the holder with the holding portion than the one side thereof, the protruding amount of the holder protruding from the one side thereof in the axial direction is made small, and the holder is made light in weight. Furthermore, the width of the cylinder head in the axial direction becomes small, whereby the cylinder head is made small in size and light in weight, this eventually leading to an internal combustion engine which is made small in size and light in weight.
Note that in this specification, when referred to xe2x80x9cwhen or as viewed from a planexe2x80x9d this means viewing from a direction of a center axis of a cylinder of the internal combustion engine.